P25 and its components - Electronic properties and photocatalytic activities

Photocatalytic activities and electronic properties of famous commercial photocatalyst - P25 has been investigated in detail. First, the active components have been isolated, i.e., anatase and rutile, characterized and tested. Although physical methods of phase separation are more recommended to keep original properties of polymorphs and to obtain the original amorphous phase, vibration and sonication-assisted vibration show to be ineffective for the phase separation. In contrast, chemical dissolution is effective, but resulting in impurification of titania surface, i.e., anatase with nitrogen and hydrogen peroxide, and rutile with fluorine. Therefore, purification methods must be applied, including thermal treatment and washing. Indeed, these post-treatment operations have resulted in preparation of almost pure anatase and rutile phases. However, annealing results in the particles’ aggregation and might activate impurities adsorbed on the titania surface. Accordingly, the isolated rutile samples have additionally been washed with NaOH (to remove adsorbed HF) and then annealed. It has been confirmed that the high photocatalytic efficiency of P25 could be explained by the intrinsic properties of anatase and rutile, i.e., high activity in oxidation and reduction reactions, respectively. The mobility of charges and lifetime of electrons, studied by time-resolved microwave conductivity (TRMC), as well as density of electron traps, investigated by reverse double beam photoacoustic spectroscopy (RDB-PAS), allow to explain the common higher activity of anatase than rutile. Accordingly, it is thought that the higher activity of rutile for hydrogen evolution might be caused either by the more negative position of conduction band level or the more efficient light harvesting ability. Interestingly, the isolated samples have exhibited the visible-light response, especially those washed with NaOH and annealed, and thus suggesting that sodium (and/or its compounds), despite being not-recommended for UV activity, might initiate vis response of titania. At the same, it should be concluded that up to now the perfect method of isolation of active phases from P25 does not exist, as each method (even followed by purification) results in some changes in the composition and/or the surface properties of anatase, rutile and amorphous phase